Which machine learning approach can be used to predict a univariate value?
Assuming your data is not purely random (otherwise it will be difficult to make useful predictions), you can try the following:


Hidden Markov Models: Treating your discrete numbers as states, you can use HMMs to predict the next state (next number) by looking at the last or the $n$ last states.
Recurrent Neural Networks: Assuming you have a lot of number sequences, you can treat your numbers as time series. Train a model and predict the next number(s). Here is a nice tutorial which uses LSTM cells.
Autoregressive models: AR models are quite powerful as well and have already been successfully used in finance and signal processing before the rise of deep learning.


Further this post (Problem in discrete valued time series forecasting) on Cross Validated might answer your question as well.